Restrict access to a post once it is tagged as a given status

Don’t remove the capability – this will make all editors unable to edit all posts. Instead, use a filter to conditionally determine if the post can be edited:

function wpse_187738_map_meta_cap( $caps, $cap, $user_ID, $args ) {
    if ( $cap === 'edit_post' && $args && ! current_user_can( 'manage_options' ) /** Only proceed for non-administrators */ ) {
        $post_id = $args[0];
        if ( has_tag( 'tag_slug_or_id', $post_id ) )
            $caps[] = 'not_allowed';
    }

    return $caps;
}

add_filter( 'map_meta_cap', 'wpse_187738_map_meta_cap', 10, 4 );